Plasma enhanced chemical vapor deposition (PECVD) of thin films is one of the most important processes used in the fabrication of very large scale integrated (VLSI) circuits. In this method, gaseous or liquid precursor chemicals are delivered to gas dispersion heads at deposition stations in a reactor chamber, where they react with the silicon substrate. If the chemical is delivered in liquid form, it passes through a vaporizer before it enters the reaction chamber. Systems for delivering chemicals to reactor chambers are a major factor in the operation of PECVD reactors. While delivery systems for gaseous precursor chemicals are well known in the art, systems for delivering liquid precursor chemicals used in PECVD are still under development.
A liquid delivery system for modern PECVD systems must meet two important criteria. First, the liquid must be delivered in a stable manner at a uniform pressure for user specified flow rates. Second, the liquid must be particle-free, gas-free and amenable to precise metering. These criteria are necessary for manufacturing advanced VLSI silicon wafers by PECVD methods. At present, pump based systems suffer from the problem of degradation of dynamic seals over time, which results in addition of unwanted particle impurities to the liquid. Also, pump based liquid delivery systems do not provide consistent delivery at uniform pressure due to the pumping forces involved in the complicated pumping machinery.
Instead of using a pump, the liquid may be supplied from a vessel containing a pressurized gas. However, in this arrangement, some of the gas may dissolve in the liquid. The presence of dissolved gases in the liquid can result in disruption of liquid flow at any downstream point. Whenever low pressure conditions are present, some of the dissolved gas may come out of the liquid in the form of bubbles. Evolution of a gas bubble in the liquid will displace a disproportionately large volume of liquid, thereby leading to inconsistent delivery. Presence of gas bubbles along with the liquid also changes the thermal conductivity of the liquid. Such changes may upset the liquid flow calibration of a liquid mass flow controller, making precise and stable delivery of liquid virtually impossible. Although other controlled orifices could be used instead of liquid mass flow controllers, in these other systems also, the presence of gas bubbles may lead to inaccuracies in metering the liquid flow. This may in turn lead to variations in the thickness and quality of the film being deposited by the PECVD process.